Baked ziti freezes really well. Be sure it's airtight in a container. If you're using plastic wrap, use a second layer of aluminum foil over that. Be sure to date it so you know when it was placed in the freezer. It'll be good 3 months.
Reheat your baked ziti in the oven. If it's frozen, let it sit in the fridge 24-48 hours before you plan to heat it. If you're using glass, let it sit on the counter first so it doesn't break by reheating too quickly.
If the slice was only refrigerated and not frozen, it should be heated through at 2-3 minutes. If it is still chilly, add it back in for 30 second intervals until heated to the temperature you like.